The subject of sentences must always agree with their verbs: both
must be singular, or both must be plural. Problems arise in the present
tense because one must add an -s or -es at the end of the verb
when the subjects performing the action is a singular third person: he,
she, it.

Notice the difference between singular and plural forms in the following
examples:

Singular

Plural

The starsings. (He or she sings)

Your grandchildrensing. (They sing)

The birdwill migrate. (It does)

Those birds migrate. (They do)

In order to find out if your subject and verb agree, you need to be able
to identify the subject of your sentence. Let's review where we find the
subject of our sentence.

Most likely, your verb will agree with the first noun to the left
of the verb which will be your subject:

The Supreme Knight decides the appropriate
penalty.

Subject: Knight

Verb: decides

OR

The council members were satisfied with the
resolution.

Subject: members

Verb: were

Sometimes, a sentence has the subject after the verb instead
of before it. This strategy is often used for poetic effect.

Over the waves roves a large heron.

Subject: a large heron

Verb: roves

OR

There was a well-known speaker at the meeting.

Subject: a well-known speaker

Verb: was

If subjects are joined by and, they are considered plural.
These are compound subjects.

The volleyball setter and the coach are having a
discussion.

Subject: the setter and the coach

Verb: are having

If subjects are joined by or or nor, the verb should
agree with the closer subject.

Either the jay-walkers or the driver is at
fault.

Subjects: driver, jay-walker

Verb: is

Either the driver or the jay-walkers are at
fault.

Subjects: driver, jsy-walkers

Verb: are

The relative pronouns (who, whom, which, and that) are either
singular or plural, depending on the words they refer to.

The pharmacist is a good researcher who spends a
great amount of time surfing the Web for information.

Subject: the pharmacist

Verbs: is, spends

OR

Pharmacists are good researchers who spend a
great amount of time surfing the Web for information.

Subject: Pharmacists

Verbs: are, spend

Indefinite pronouns (someone, somebody, each, either one,
everyone, or anyone) are considered singular and need singular
verbs although they convey plural meaning.

Anyone who wants to pursue higher education has
to pass entrance exams.

Subject: anyone

Verbs: wants, has

OR

Everyone on the team is welcome to express
his/her ideas.

Subject: everyone

Verb: is

A few nouns can be either plural or singular,
depending on whether they mean a group or separate individuals. These
words are rarely used as plurals in modern writing.